1. Field of the Invention
The present invention relates a non-aqueous electrolyte and a lithium secondary battery using the same.
2. Description of the Related Art
In recent years, use of compact electronic devices such as personal computers, cellular telephones, camcorders, has remarkably spread. As the sources of power for driving these compact electronic devices, compact, light weight, and high capacity secondary batteries are in strong demand.
From these perspectives, a lithium secondary battery having a complex oxide such as lithium cobalt dioxide, lithium nickel dioxide, or lithium manganese oxide as a cathode active material, a carbonaceous material capable of doping and undoped with lithium ions as an anode active material, and a non-aqueous electrolyte comprised of a non-aqueous solvent in which is dissolved a lithium salt has been considered suitable. Research and development are progressing rapidly with the aim of further improvement.
Among the carbonaceous materials capable of doping and undoping the same with lithium ions, graphite is one of the optimum compounds as the anode active material for a lithium secondary battery since it has the characteristics of (1) a large electrical capacity and (2) a high flatness of electric potential.
However, if cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) are used as the electrolyte solvent in a lithium secondary battery using a graphite-based material as an anode active material, these carbonates are decomposed by the graphite-based anode active material. At that time, exfoliation of the carbonaceous material will occur and the battery characteristics such as electric capacity, cycle characteristics, storage characteristics, are decreased. In particular, this phenomenon remarkably appears with an electrolyte containing PC. At the time of initial charging, the PC is decomposed by the graphite anode and therefore, charging-discharging becomes impossible.
As methods for suppressing the decomposition of the cyclic carbonates in the electrolyte by the graphite-based anode active material and the exfoliation of the carbonaceous material, various additives have been proposed. For example, J. Electrochem. Soc., vol. 140, no. 6, L101 (1993) proposes the addition of crown ether (12-crown-4) to an electrolyte having a PC and EC base. In this case, however, unless adding a considerably large amount of the expensive crown ether, the effect of suppressing the decomposition is small and the battery characteristics are still insufficient, and therefore, practical problems remain. Further, Japanese Unexamined Patent Publication (Kokai) No. 8-45545 (i.e., U.S. Pat. No. 5,626,981) discloses the addition of vinylene carbonate (hereinafter referred to as xe2x80x9cVCxe2x80x9d) so as to suppress the decomposition of an electrolyte having a PC or EC base. According to this method, the additive is reduced by the anode at the time of charging, a passivation film is formed on the surface of the graphite, and the reduction of the other solvents such as PC, EC are suppressed.
However, the initial coulomb (charge-discharge) efficiency is not necessarily high with these methods. In addition, repeated charge-discharging causes the electric capacity to gradually drop, and therefore, a satisfactory cycle characteristics and storage characteristics cannot be obtained.
1997 Joint International Meeting of the Electrochemical Society, Inc. and International Society of Electrochemistry, Abstracts, p. 153 (1997) reports that in voltamogram measurement using a cell having an electrolyte containing 5% by volume of VC and a 1M LiPF6, and having a volume ratio of PC/EC/DMC (where DMC indicates dimethyl carbonate) of 1/1/3 and having a graphite electrode (working electrode)/Li (counter electrode)/Li (reference electrode), a reduction peak appears at 1 V and this forms a passivation film on the anode and suppresses the reduction of the other solvents.
Further, J. Electrochem. Soc., vol. 140, no. 9, L161 (1995) states that the addition of chloroethylene carbonate to an electrolyte suppresses the decomposition of the PC at the surface of a graphite electrode. This is believed to be because the decomposition products of the chloroethylene carbonate form a passivation film at the surface of the graphite. However, the effect of suppression of decomposition of the electrolyte is not necessarily good.
While the above methods enable the use of cyclic carbonate at highly crystalline carbon anodes such as graphite, the battery characteristics are still not sufficient.
The objects of the present invention are to solve the above-mentioned problems in the prior art and to provide a non-aqueous electrolyte capable of providing secondary battery having the electric capacity of a lithium battery and superior cycle characteristics and storage characteristics and to a lithium secondary battery using the same.
In accordance with the present invention, there is provided a non-aqueous electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein and a cyclic carbonate, linear carbonate, and a vinylene carbonate having a chlorine content of 100 ppm or less.
In accordance with the present invention there is also provided a lithium secondary battery comprising a cathode, a graphite anode having a lattice spacing (d002) of 0.34 nm or less, and a non-aqueous electrolyte comprising a lithium salt dissolved in a non-aqueous solvent, wherein the non-aqueous electrolyte contains, as the electrolyte, a cyclic carbonate, linear carbonate, and vinylene carbonate having a chlorine content of 100 ppm or less.
The present inventors made an intensive research on the effects of VC on suppression of the electrolyte decomposition at the surface of a graphite electrode, and learned that an electrolyte including VC produced by the conventional method did not have a satisfactory battery characteristics and further that it did not have a reproducible data in the battery characteristics. Further, we engaged in repeated studies and, as a result, found that VC produced by the conventional method inevitably contained a considerable amount of organic chlorine impurities, which caused a deterioration in the battery characteristics and it did not have a reproducible data in the battery characteristics.